1. Field of the Invention
The present invention generally relates to ball grid array (BGA) and in particular, an apparatus and method for filling a ball grid array template with solder balls.
2. Description of the Prior Art
Ball grid arrays techniques are common nowadays for connecting high-density IC components onto circuit boards. U.S. Pat. No. 5,655,704, issued to Matsushita Electric Industrial Co., 12 Aug. 1997 discloses a method of filling a template or a stencil with solder balls as a sweeping bin passes over the template. However, such method is very susceptible to solder ball damage by the design itself as well as due to process variability, contamination or foreign particle. As a result of the damage, subsequent process becomes not possible or the object of subsequent depositing the solder ball on the BGA package is not possible.
FIG. 1 shows a conventional method of filling a BGA template with solder balls. The BGA template 1 is provided with a plurality of holes to receive a plurality of solder balls 3. A ball bin 2 sweeps over and above the template 1 and deposits the solder balls 3 into the holes on the template 1. As this deposition of the solder balls 3 can happen anytime when the solder ball 3 and the hole are aligned, the deposition may take place just as the ball bin 2 is about to reach the particular hole at which the solder ball 4 is just about to drop in. As shown in FIG. 1, this may result in the ball 4 being quashed by the bin 2. Another possible situation is when the solder ball 6 fails to properly seat in the hole due to foreign matter 5. The bin 2 will shear off the top of the solder ball 6.
FIG. 2 is a perspective view showing a solder ball mounting apparatus disclosed in U.S. Pat. No. 5,655,704. As shown in the FIG. 2, the solder balls are deposited onto the substrate 10 through the stencil 7. The flux dots 11 on the substrate hold the solder balls 9 in place. However, as these flux dots 11 are liquid paste, the volume or height of the dot 11 may vary as in the case of flux dot 12, where there is excessive amount. This causes the solder ball 13 to stick out of the stencil 7 and be sheared off at the top as the solder ball bin 8 passes over the ball 13. There can potentially be cases where there is not any flux dots and the solder balls will drop lower than the usual height and the solder ball 14 will be sheared by the bin 8.
U.S. Pat. No. 5,499,487, issued to Vanguard Automation, Inc., 19 Mar. 1996 discloses an apparatus for placing solder balls in a ball grid array. The apparatus comprises a wheel having an inner and an outer face, and the wheel being rotatable about a horizontal axis, means for attaching a ball grid array to the inner face of the wheel, means for attaching a tooling fixture to the outer face of the wheel in a position corresponding to that of the ball grid array, means for forming a reservoir of solder balls at the bottom of the wheel, means for controllably rotating the wheel to move the tooling fixture through the reservoir in a manner to fill recesses in the fixture with solder balls and to remove from the surface of the array any excess solder balls which are not occupying recessed, the inner and the outer faces being separated a distance to permit the tooling fixture to engage solder balls in the reservoir while ball grid array does not engage the solder balls.
U.S. Pat. No. 5,551,216 relates to a method and apparatus for filling a ball grid array. The apparatus comprises a reservoir for solder balls and a tooling plate with an array of holes for receiving solder balls. U.S. Pat. No. 6,276,598 relates to a method and an apparatus for ball placement. The method and apparatus are particularly suitable for transferring solder balls to a ball grid array package.
The methods disclosed in the above U.S. Pat. Nos. 5,499,487, 5,551,216, and 6,276,598 make use of free flow of solder balls 17 over tilted templates 15 with or without vacuum suction at the holes receiving the solder balls. This method does not prevent excess solder balls 18 and solder balls 19 remaining on the surface of the template or around the holes due to either static forces, vacuum leakage at holes or foreign matter which may be sticky like flux used in the attachment to the substrate. In these citations, subsequent operation whereby the ball pick head 16 or a ball grid array substrate 20 is advanced above the template will result in quashed solder ball or multiple balls at each hole in the pick head or at each electrode on the ball grid array (as shown in FIGS. 3 and 4).
U.S. Pat. No. 5,918,792, issued to RSVI Vanguard, Inc., 6 Jul. 1999, discloses ball grid arrays and method for placement of solder balls onto the pattern of metallized pads or dots of such arrays. The apparatus comprises an elongated holder defining an elongated recess therein for the carrier of the array, a stencil overlying the recess and supported by guide rails, a block of rigid material having a planar bottom surface contacting the stencil and for movement along the stencil, and means for moving the block along the stencil for moving a bin contacting solder balls over the apertures formed on the stencil for filling the apertures.